linux-kernel - Re: [PATCH] serial: 8250: drop lockdep annotation from serial8250_clear

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Date:   Tue, 15 Aug 2023 12:10:17 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        gregkh@...uxfoundation.org, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] serial: 8250: drop lockdep annotation from
 serial8250_clear_IER()

On 15. 08. 23, 11:27, Petr Mladek wrote:
> On Mon 2023-08-14 12:21:23, Jiri Slaby wrote:
>> On 14. 08. 23, 12:00, Petr Mladek wrote:
>>> I personally vote to keep it as is unless people see this warning
>>> on daily basis. After all, the lockdep splat is correct. The serial
>>> console might not work correctly in panic() when there is the race.
>>
>> Sorry, but no, the warning is not correct at all. The code path deliberately
>> does NOT take the lock and calls a function which is currently annotated
>> that the lock is _always_ taken. Therefore, the warning is clearly a false
>> positive and I see no reason in keeping it.
> 
> There might be a misunderstanding. I only want to keep panic()
> implementation as it is for now. I mean to keep calling
> debug_locks_off() right before console_flush_on_panic().
> The lockdep should stay on before to report potential problems
> in non-printk code, like kexec, panic notifiers.
> 
> But I am fine with disabling the particular lockdep_assert_held_once()
> during panic().
> 
> It should stay during the normal system state to catch not
> yet discovered races. John is working hard on preventing any
> races which might blow up after introducing the printk kthreads.
> 
> I mean something like:
> 
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index ecfdc4534123..9533c1eedfb1 100644
> --- a/drivers/tty/serial/8250/8250_port.c
> +++ b/drivers/tty/serial/8250/8250_port.c
> @@ -704,7 +704,8 @@ static void serial8250_set_sleep(struct uart_8250_port *p, int sleep)
>   static void serial8250_clear_IER(struct uart_8250_port *up)
>   {
>   	/* Port locked to synchronize UART_IER access against the console. */
> -	lockdep_assert_held_once(&up->port.lock);
> +	if (!oops_in_progress)
> +		lockdep_assert_held_once(&up->port.lock);

Yes, this is one of my suggestions ;). (Which I thought are not worth 
it, but I am not opposing either.)

thanks,
-- 
js
suse labs